In the semiconductor industry and especially thin film semiconductor devices such as thin film transistors (TFTs), the devices include spaced apart source and drain areas that conduct through a channel layer positioned therebetween. At least one gate insulator and gate electrode are positioned above and/or below the channel layer, to control the conduction. In many applications TFTs are used where high heat cannot be tolerated during fabrication and, thus, a semiconductor must be used that can be deposited at relatively low temperatures (e.g. room temperature) but which still has relatively high mobility.
There is a strong interest in metal oxide semiconductor because of its high carrier mobility, light transparency and low deposition temperature. The high carrier mobility expands applications to higher performance domains that require higher frequency or higher current. The light transparency eliminates the need for a light shield in display and sensor active matrices. The low deposition temperature enables application to flexible electronics on plastic substrates.
The unique features of metal oxide semiconductors are: (1) carrier mobility is less dependent on grain size of films, that is, high mobility amorphous metal oxide is possible; (2) density of surface states is low and enables easy field effect for TFTs, this is contrary to covalent semiconductors (such as Si or a-Si) where surface states have to be passivated by hydrogen; and (3) mobility strongly depends on the volume carrier density. In order to achieve high mobility for high performance applications, the volume carrier density of the metal oxide channel should be high and thickness of the metal oxide film should be small (e.g. <100 nm and preferably <50 nm).
However, a major deficiency of metal oxide semiconductors is stability and the tendency to become polycrystalline at higher process temperatures. Popular metal oxides, such as zinc oxide, indium zinc oxide, and indium gallium zinc oxide, are not very stable and become polycrystalline at moderate process temperatures (e.g. greater than approximately 400° C.) Polycrystalline semiconductor metal oxides are not desirable in semiconductor devices for several reasons. For example, the characteristics of transistors formed in polycrystalline semiconductor metal oxides can vary, even between adjacent devices in an array, because of the variation in crystal size and position. To better understand this problem, in a conduction area under a sub-micron gate each different transistor can include from one or two poly-silicon crystalline grains to several crystalline grains and the different number of crystals in the conduction area will produce different characteristics. The dimensions and their physical characteristics among different grains are also different.
The stability of metal oxide thin film transistors (TFTs) depends strongly on processing temperatures. At high temperatures, the traps in the bulk semiconductor layer and at the interface or interfaces between the gate insulator and the semiconductor layer can be reduced. For applications, such as active matrix organic light emitting devices (AMOLED), extreme stability is required. It is advantageous to take the metal oxide TFTs to high temperatures, generally between 250° C. and 700° C., during processing. Meanwhile it is desirable to maintain the amorphous nature of the metal oxide at these processing temperatures.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved metal oxide semiconductor material.
Another object of the invention is to provide a new and improved metal oxide semiconductor material with improved stability and has less tendency to become polycrystalline at higher processing temperatures.
Another object of the invention is to provide a new and improved metal oxide semiconductor material with improved stability, high carrier mobility, and good control of oxygen vacancies and carrier density.